Has Ed Balls got what it takes?

Much has been made of the difficulty Des Browne must have in combining his two jobs as Defence Secretary and Scottish Secretary. More worrying still is the dual role played by Ed Balls.

The Secretary of State for Children, Schools and Families should be free to spend every waking hour he has driving up standards in education - but instead he is also expected to act as the Prime Minister's hand-holder-in-chief.

Today, Mr Balls will publish the Government's Children's Plan, which will seek to ensure that the next generation (and their parents) are on the right track by reforming primary school testing, giving free nursery places to two-year-olds and tackling the commercialisation of childhood.

It will be a hugely ambitious package, the first sign of the direction in which Gordon Brown intends to move the public services - and a key test of whether the Prime Minister can convince voters in Middle England that he is on their side.

But, in recent weeks, the man responsible for drawing up the plan has also had to deal with Northern Rock, missing CDs and the donor row. Before that, he was busy preparing for (and, indeed, hyping up) the election-that-never-was.

According to exasperated officials and ministers in his department, he can be called away to Downing Street three or four times a day. There are often many more phone calls, text messages and emails to handle from an anxious PM. Every morning at seven, he has to participate in a conference call with key Number 10 aides to discuss the strategy for the day.

He is sent polling results, focus group research and media briefs for every subject under the sun. It is a miracle he has a moment left to save Britain's failing schools.

Mr Balls is phenomenally bright - he talks quickly, absorbing ideas as rapidly as he pours them out. A 20-minute coffee with him is more rewarding than a two-hour lunch with most ministers.

Last week, he seemed remarkably relaxed when he appeared momentarily moonlighting as Father Christmas at a children's party in Number 11. When one child piped up that he wanted a new pair of shoes in his stocking, Santa replied, quick as a flash: "Get your parents to use their child benefit to buy them." "It's lost in the post," the parent shouted back.

But with three young children, a wife - Yvette Cooper - who has a seat alongside him at Cabinet and two homes to shuttle between, the sleep-deprived 40-year-old knows how difficult juggling can be.

He once had the family's online supermarket food shop delivered accidentally to the wrong address. The bags were outside the London home, the children in the Yorkshire constituency - and hungry.

As a politician, he needs to make sure that something similar does not happen, metaphorically, as he rushes between his office in Great Smith Street and Number 10.

The scope of the newly created Department for Children, Schools and Families is enormous - it covers everything from SATS to fat, from drugs to hugs. It is responsible for the work-life balance and children's television, as well as reading, writing and arithmetic.

Education is Mr Brown's self-declared passion. Politically, the department's wider remit is a crucial battle zone for the PM, who is struggling to prove that he, like David Cameron, understands that "general well-being" is as important as GDP.

The Secretary of State should be able to concentrate on his day job rather than having to help Mr Brown trouble-shoot as well.

As Mr Balls said in an interview on Sunday, he has spent 15 years working as Mr Brown's closest aide, so it is not surprising that the Prime Minister still turns to him for advice.

But it is also a sign of Mr Brown's insecurity that he is so heavily reliant on the small circle with whom he has been drinking beer, watching football and devising policy for so long.

Other ministers are beginning to resent the fact that Tony Blair's sofa has been replaced by what could be called an "armchair" style of Government - even smaller, but not as cosy.

Mr Balls is in danger of becoming the lightning conductor for a growing sense of discontent about Mr Brown - Labour MPs like to quote Michael Heseltine's quip that, "It's not Brown, it's Balls". He is the most powerful, and therefore the most resented, of what one former minister yesterday called the "unwise boys" around the Prime Minister.

At times, he can be arrogant - at the Treasury, he would march, unannounced, into meetings Mr Brown was holding with ministers and tell them he disagreed with everything they said.

His tribalism is well known - according to a recent BBC documentary, he told Labour rebels who were causing trouble for Mr Blair last year to "keep up the good work". "He's Gordon's creation," says one minister. "He's a very good apparatchik, but that's it."

Now, however, Mr Balls has made the transition from back-room operator to front-room politician, and he should have the opportunity to prove himself there, rather than being constantly ushered back behind the scenes. It is in Mr Brown's interest to widen his circle - and it is also, of course, in Mr Balls's own interest to show that he can stand on his own two feet.

Increasingly, the chatter in the Commons tea rooms is turning to the leadership contest that will unfold when Mr Brown eventually stands down. According to one close ally of the Prime Minister, Mr Brown has made clear, privately, that he does not intend to fight more than one election. I presume that, if this is true, he has also made the position clear to Mr Balls.

Some MPs believe that the Children's Secretary is already positioning himself as the "True Labour" candidate who would stand against the more "New Labour" David Miliband. His department has, for example, announced a review of city academies (a bête noire for the Labour Left), although it is unclear where it will lead.

In the past, Mr Balls has said he prefers the label "socialist" to the more New Labour "social democrat". And it was in fact he who first argued, in an interview five years ago, that there were "limits to the market" in the public services (a point more famously repeated three months later by Mr Brown in a speech).

The beauty contest is not yet in full swing but it is possible occasionally to get a flash of swimsuit from beneath the grey suits. It is not only the Prime Minister's fate that is resting on the success of today's Children's Plan.